Photoselective ionization

The resolution of photoion laser microscopy is limited by two fundamental factors [7] the Heisenberg principle of uncertainty and the presence of the nonzero tangential component of the velocity of the ejected photoion (photoelectron). The same factors restrict the spatial resolution of the field-ion microscopy. It must be emphasized again that the key difference lies in the fact that for photoion microscopy there is no need for a strong (ionizing) electric field that distorts and desorbs the molecules. And also, the femtosecond laser radiation allows the photoion to be photoselectively extracted from certain parts of a molecule. 

Ionization Nonselective by strong electric field Photoselective by laser pulses... 

The third class of mechanisms, involving either direct ionization of the liquid or electron ejection via field-emission, has been used to study the behavior of quasifree, localized, and solvated electrons. In contrast to the photoselectivity of the previous two schemes, a cascade of events occurs when high-energy electrons impart energy to a liquid. The resulting ions, excited states, and excess electrons provide a complex spectrum to unravel. However, the temporal evolution of each of the various species differs significantly and we are able to focus on the primary picosecond event, electron localization, with little interference. 

The goal of this book is to present in a coherent way the problems of the laser control of matter at the atomic-molecular level, namely, control of the velocity distribution of atoms and molecules (saturation Doppler-free spectroscopy) control of the absolute velocity of atoms (laser cooling) control of the orientation, position, and direction of motion of atoms (laser trapping of atoms, and atom optics) control of the coherent behavior of ultracold (quantum) gases laser-induced photoassociation of cold atoms, photoselective ionization of atoms photoselective multiphoton dissociation of simple and polyatomic molecules (vibrationally or electronically excited) multiphoton photoionization and mass spectrometry of molecules and femtosecond coherent control of the photoionization of atoms and photodissociation of molecules. 

Photoselective ionization combined with mass spectrometry seemed from the outset to hold the greatest promise for the study and identification of complex organic molecules, especially biomolecules. This is due to the fact that two-photon ionization at moderate laser pulse powers causes no strong fragmentation of the molecules, whereas ionization by electron impact causes substantial fragmentation. This is evident from a comparison between the mass spectra presented in Figs. 10.4(a) and (d). Several ways have been found to achieve this goal by means of soft laser desorption of molecules from a surface, followed by their photoionization, specifically in a jet-cooled stream, and chemiionization in a dense cloud of desorbed (ablated) biomolecules. 

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